1. Technical Field
The present invention relates generally to apparatus used in man-made underground installations, and more particularly to apparatus, such as manhole covers and drain grates, which cover surface openings to such underground installations.
2. Background Art
Manhole covers are among the oldest of commercial products. They are not exempt, however, from the changes being wrought by our modern culture. Most notably, (1) the quality revolution, (2) sociological pressures to make products more ergonomically acceptable to women and the handicapped, and (3) safety concerns for workers entering confined spaces such as manholes.
The quality revolution is leading firms to produce products better suited to the end user at the lowest possible cost. In the case of manhole covers, the goal is to make them easy to remove and handle (low weight), and to use the least amount of material consistent with strength requirements (low weight). In general, consulting engineers and municipal engineers specify the manhole cover designs used in their areas of responsibility. They desire peace of mind that no manhole cover will ever fail in service. Until now, they have relied on historical evidence and proof load tests to assure design strength. Neither method provides rigorous evidence of design adequacy, and neither allows for good value engineering which is necessary to succeed in the quality revolution.
Women are now undertaking careers that have been traditionally held by men. Jobs in construction and maintenance of underground installations, such as sewers and drains, are no exception. Such jobs require the handling of relatively heavy manhole covers which expose any worker, male or female, to the possibility of personal injury. But, with the increase of women in these types of jobs, there has arisen a greater need to reduce the weight of manhole covers.
In some applications, it is desirable to construct a manhole with an opening as large as possible. A large manhole facilitates entry into, and exit out of the installation, especially when the worker is carrying equipment and tools, utilizing breathing apparatus, evacuating disabled workers, or is large in stature. In addition, large manhole openings facilitate the cleaning of underground installations, such as grease traps. However, larger diameter manholes obviously require larger and heavier manhole covers. Thus, there is a need for a manhole cover design which is optimized to reduce the weight of the cover for a given strength requirement (i.e., maximize the strength-to-weight ratio). With such an optimized design, larger manhole covers could be utilized without exposing the worker to an undue risk of injury.
There are two basic types of manhole covers in use today--(1) ribbed covers, and (2) platen covers. Ribbed covers are older, and more traditional in design. They utilize stiffener ribs in concentric circles, radial patterns, or square patterns. There is very little deflection in these covers. The problem with these covers is that less material is located in areas subjected to tension. Grey iron, the most commonly used material for manhole covers, is about three times stronger in compression than in tension. Thus, a ribbed design is the worst choice if grey iron is selected as the material for the cover.
In addition, the stiffeners in ribbed covers are not efficient in a strength-to-weight sense. Ribbed covers do not lend themselves to rigorous value engineering design. The stiffeners in ribbed covers also limit energy absorption. The ability of a manhole cover to absorb energy is determined by the amount of material subjected to bending. As indicated above, there is very little bending in a ribbed cover. Thus, a ribbed cover is more prone to failure, especially when subjected to overload conditions.
Platen covers were introduced in the last two decades. A platen cover has a uniform thickness, except for the annular bearing ring around the periphery of the cover. Platen covers are of a monolithic construction. They provide strength-to-weight characteristics which are improved over ribbed designs, because they have more material in areas of tensile stress. The monolithic design also reduces stress concentrations that contribute to fatigue failure. However, rigorous value engineering is very limited with platen covers, because the designer can only adjust the uniform thickness.